In this post, Lalla writes: If the solubility is 0.5% at 25°C, does it mean if we dissolve a higher concentration at say 35°C the allantoin will crystallize as it cools down? If I try to dissolve MSM and allantoin and caffeine in water, will their respective solubility decrease because of the other ingredients?
Solubility is generally measured at STP, which is standard temperature and pressure. The official IUPAC STP is 0˚C and 100 kPa (the pressure at sea level. We generally use SATP or standard ambient temperature and pressure, which is considered to be 25˚C or 77˚F and 100 kPa (sea level).
If something has a solubility of 0.5% at SATP, this means at 25˚C you can dissolve 0.5 grams of it in 100 grams of solvent (alcohol, water, oil, and so on). So let's say you have something like green tea extract and its solubility is 0.5% at SATP, this means that I should be able to dissolve 0.5 grams of green tea extract in 100 grams of water at 25˚C. If it cools down below 25˚C, the solubility might go down slightly, but not enough that we have to worry about it!
When we increase the temperature, we generally increase the solubility of the ingredient, so we can dissolve more of it. I can get far too much powdered extract to dissolve in my toner when it's warm (45˚C or slightly lower), but it'll precipitate when it cools down (see the picture above). Which is why we want to make sure we always stay at the suggested usage rates!
One exception to this increase the temperature, increase the solubility is carbon dioxide in pop. When we cool it down, it gets more soluble, which is why a pop from the fridge feels fizzier than that from your shelf! Interesting, eh?
If you're making something like a toner, you'd be surprised at how much you can add before you get a precipitate! Water is a great solvent for things that like to be dissolved in water, and you could get quite a bit in there. I've made toners that contained a ton of things - 0.5% allantoin, 0.5% green tea extract, 0.5% chamomile extract, 0.5% grapeseed extract, 2% niacinamide, 2% MSM, 2% salicylic acid (what's that - 8% powder?) and they stayed nice and solubilized, and the product stayed liquid!
I will point out that this one looks kinda muddy, and it isn't going to be clear at any point in its lifetime because I've loaded it up with tons of great extracts and powders. But the question was whether the various powders would have an effect on the solubility of other powders, and the answer in general is no, they shouldn't if we're using things at suggested rates!
Here's a toner filled with tons of water soluble powders, and here's another!
Did I answer your question? Join me tomorrow for more fun with cosmetic chemistry!